Conventionally, ethane and naphtha (boiling point less than 200 degrees Celsius (deg C.)) can be used to produce light olefins, such as ethylene and propylene by a steam cracking process. Naphtha, such as straight run naphtha, can be a paraffin-rich hydrocarbon stream. However, the naphtha fraction in crude oil can be limited to between 10 and 30 percent by volume (vol %). Conventional steam cracking processes can be used to process gas oil. Gas oil refers to hydrocarbons with a boiling point between 200 deg C. and 565 deg C., where light gas oil has a boiling point between 200 deg C. and 370 deg C., and where vacuum gas oil has a boiling point between 200 deg C. and 565 deg C. Due to the presence of heavier molecules steam cracking of gas oil results in a reduced yield of ethylene and propylene and greater coking rates relative to steam cracking of naptha.
In petroleum-based crude oils, various types of molecules are present. Per their chemical structure, molecules can be classified as paraffin, olefin, naphthene (a cyclic paraffin), and aromatic, which can be indicated by an oil composition's PONA number. Of the molecules, it is most efficient to convert paraffins to light olefins, with n-paraffins being more effectively converted than iso-paraffins, and for this reason n-paraffins can be preferred. Aromatics, such as benzene and toluene, are stable at high temperatures, have a low hydrogen to carbon ratio, and are known to be an effective precursor for coke formation. The stability of aromatics can be contributed to the carbon-carbon bond energies of the aromatic carbon, as compared to the carbon-carbon bond energies of paraffinic carbons. For these reasons, aromatics are difficult to convert and are not a good source for producing light olefins. Naphthenes as a source material are more difficult to convert than paraffins, but easier to convert than aromatics. Olefinic compounds are generally cracked in a steam cracker to produce light paraffins and olefins while aromatics with longer-chained olefins are formed through cyclization followed by dehydrogenation reactions. Thus, a feedstock for a steam cracking process preferably contains a majority n-paraffins, followed by iso-paraffins, and naphthenes with little or no olefins or aromatics.
Steam cracking processes cannot effectively process heavy fractions that contain asphaltene. Steam cracking asphaltene can produce coke, which can result in plugging of the process lines.
Some pre-treatment steps can be taken to make gas oil or other heavy oils suitable for use as a steam cracking process feedstock. Pre-treatment approaches can include hydrotreatment, thermal conversion, extraction, and distillation. Extraction processes can include a solvent deasphalting process. However, these processes produce liquid yields of less than 80 percent by volume, resulting in analogous low product recovery from the steam cracking process. In addition, pre-treatment processes can increase the cost per barrel to the resultant products.